Electrodeposition monitor



1967 F. J. SCHMIDT ELECTRODEPOSITION MONITOR 3 Sheets-Sheet 1 FiledMarch 1967 Fly.

INVENTOR.

FRANCIS J. SCHMIDT BY/7IWZ11 W/fwflm AGENT Dec. 5, 1967 Filed March 2,

ELECTRODEPOSITION MONITOR 3 Sheets-Sheet 2 '5. O I; E5

| I z I q- I I I A" II H "I 8 o n BF a P- 0F a a a I!) ID ID 0 m 0 OINVENTOR.

FRANCIS J. SCHMIDT WM 0 WW AGENT Dec. 5, 1967 F. J. SCHMIDTELECTRODEPOSITION MONITOR 3 Sheets-Sheet 5 Filed March 2, 1967 INVENTOR.FRA/vc/s J. SCH/WOT, 5v fi MV V WWW AGENT United States Patent ABSTRACTOF THE DISCLOSURE Stress in electrodeposit formed at given currentdensity is determined by feeding strip electrode from supply roll intoelectrodeposition bath, and depositing upon immersed portion of strip atcurrent density of interest. Stress is determined by measuring byinstrumental means the resulting displacement of strip, which bowsinward or outward as result of stress in deposit. Current densityapplied to work pieces in same bath may be adjusted in accordance withinstrumental measurement of stress to produce desired value of stress indeposits on work pieces. Fresh portion of strip is provided by advancingstrip from supply roll, taking up used portion on takeup roll.

Reference to copending application This application is acontinuation-in-part of my copending application Ser. No. 326,706, filedNov. 29, 1963, entitled Electrodeposition Monitor, now abandoned.

Specification This invention pertains to the art of electrodeposition,and more particularly to the monitoring and control of the currentemployed in electrodeposition to produce a coating having predeterminedinternal stress (or lack of stress).

In the art of electrodeposition, and particularly the art ofelectroplating, it is Well known that the particular current densityemployed is one of the parameters which determine whether the depositproduced is in a state of internal tension, compression, or whether itis substantially stress-free. Ordinarily a stress-free deposit isdesired, although there are certain conceivable situations in which aninternal stress of given sign and magnitude may be desirable. Thepresently conventional means of adjusting the current to produce thedesired state in the deposit is largely empirical. In my copendingapplication for US. Patent, Ser. No. 437,623, Feb. 18, 1965, parent Ser.No. 246,743, Dec. 24, 1962, now abandoned, entitled Device for MeasuringElectrofinishing Stresses, and assigned to the assignee of the presentapplication, there is disclosed a device and the mode of its applicationto determine which particular value of current density produces a giveninternal stress in the coating. This, briefly, is accomplished bydepositing coatings upon a conductive substrate of known thickness andelastic characteristics, and observing the bending of the substrate whenthe desired thickness of deposit has been applied. The current in themain or production electrodeposition apparatus may then be adjusted toproduce the current density which has been found to produce the desiredresult. However, in large production deposition baths the compositionand deposition characteristics may change with use, both by depletionand by accretion of impurities. Since these changes occur gradually,during use of the bath, it is highly desirable that some means to beprovided to monitor and adjust the current automatically to maintain thedesired deposit characteristics, since unwanted stresses in deposits mayproduce defects such as peeling and susceptibility to corrosion which,unfortunately, are not immediately obvious, but appear only during thelife of the coated part. When it is considered that parts used in largequantities, such as auto- "ice mobile bumpers, are electroplated incontinuous plating baths of the kind described, it is evident that theelimination of defective production is of economic importance evenbeyond the value of the plated part, since defects in such plated partsmay impair the reputation of a much more expensive product of which theplated item is only a minor but conspicuous or essential part.

This specification describes a device for automatically controlling thecurrent in a continuous plating operation to minimize (or otherwiseadjust) stress in the deposited coating. It may also be used todetermine the effect upon the appearance or other characteristics of thecoating of variations in current density.

In one embodiment of my invention, a cell is provided which iselectrically similar to the type of cell whose basic properties areemployed in an embodiment of my referenced invention for measuringstress in electrodeposited coatings. However, the test electrode, whichin most processes is the cathode, instead of consisting of a flat metalsheet which is divided by slits into a number of separate tongues, asdisclosed in my copending application, is composed instead of aplurality of strips of metal which are fed continuously from top tobottom over guides and thereby constitute the test electrode.

A number of these strips, located vertically side by side present asurface which is electrically similar to the continuous surface of asingle electrode or, even more nearly, to the individual tongues of theelectrode disclosed in my copending application. While the individualstrips are maintained approximately vertical in their progress downwardthrough the bath, they are given freedom to move slightly in response tocurvature resulting from tension or compression stresses in thedeposited coating. This is accomplished by stretching the strip betweentwo sheaves or rollers which are located one vertically over the other,the strip thus being free for small lateral displacements. In accordancewith the known art and also the teachings of my invention earlierreferred to, if the electrodeposited coating is in tension, the stripwill tend to be bowed in the direction of the coating, i.e., it will beconcave on that side. If the coating is in compression, the strip willbe bowed around a center away from the coating, that is to say, so thatit will become convex on the side carrying the coating. The currentdistribution in such a plurality of anode strips will be substantiallythe same as that in the conventional Hull cell; that is to say, thecurrent density to electrode strips nearer to the counterelectrode isgreater than that to electrode strips farther away, the counterelectrode being inclined at an angle to the faces of the strips. Thedeflection of such a strip will not be so great as to produce any markedchanges in the current density to the strip. To measure such a smalldisplacement advantage may be taken of the conductivity of the strip byplacing a solenoidal winding behind the strip, preferably with its axisnormal to the strips surface, and measuring the inductance of the coil.If the strip is displaced toward the winding, it will, in general, tendto reduce the inductance. This assumes that the strip is electricallyconductive and that any ferromagnetism of the coating is negligibleeither because of the frequency employed or because of the thinness ofthe coating. If the permeability of the strip or of the coating issufiiciently high, the inductance may be increased. In any event, itwill be changed. A convenient and standard way of measuring changes inthe inductance of such a coil is to determine changes in the frequencyof an oscillator in which the coil forms part of the oscillatingcircuit. It is evident that a variety of methods may be used todetermine the amount by which the strip has been displaced.

After downward passage through the field associated with the cell testelectrode region, the strip may be drawn upward through the bath forfurther examination and be either stripped of its coating and reused or,as is likely to prove more practical, be simply discarded. It is truethat there will be some current flow to the strip in its upward passage.It is assumed that this will be negligible because of the greaterdistance of the strip at that time from the Hull cell anode. If, in agiven design, this should prove not to be true, an insulating sleeve maybe inserted in the bath to lengthen the path of flow between thecounterelectrode and the strip. This can be made adequate to reduceunwanted currents.

The strip which has been removed from the bath may be inspectedphysically for appearance, or may be tested for the determination ofvarious factors which can be determined by instruments, such assmoothness or thickness of coating.

The indications of the deflection of the strip during its passagethrough the cell may be employed in several ways. They may simply betransmitted to a human operator who will use the information to decidewhat current density is required to give the deposit that he desires.Alternatively, the outputs of the devices for measuring the displacementof the strip may be fed into some data processing device whose result isused to adjust the bath current directly. This may conveniently be meansfor selecting the particular strip (or deflection sensor) associatedwith the desired value of deflection, and control means responsive tothe output of the selected deflection sensor to adjust the current.

The preceding description is of the most general version of myinvention, in which a plurality of strips is employed to determinestresses resulting at a plurality of different current densities. It isapplicable to electrodeposition operations generally in which coatingstress may, with variation in current density, increase for a while andthen decrease, then increase again, possibly crossing the stress axisseveral times. For those particular baths in which, over the workingrange of cur-rent densities, the stress varies monotonically, it ispossible to use a single strip which is plated at the current densityactually in use on the work pieces. The deflection of such a strip willindicate the direction in which the current density should be adjustedto produce the desired result, and, by sufficiently frequent orcontinuous observation of the deflection of the strip and currentdensity adjustments, the current density may be caused to oscillate withadequately small amplitude around the desired value, withcorrespondingly small deviations of deposit stress from the desiredvalue. This simplified scheme, while less elegant and somewhat lessaccurate than the more elaborate multi-strip approach, is simpler andcheaper.

Thus I achieve a number of novel and useful objects. I make possible thecontinuous monitoring, and the automatic control, of the current densityemployed in electrodeposition to maintain desired depositcharacteristics despite variations in the characteristics of the medium,or bath, employed. Achievement of this object produces economy andimprovement in product characteristics. I also reduce the use of skilledlabor in supervising the operation. I further produce a continuoussample, easily retained for reference, of the quality of depositproduced during the operation of the bath over prolonged periods oftime. My invention produces various other benefits and useful advantageswhich will become apparent to those skilled in the art, in the course ofthe further specification and description.

For the better understanding of my invention, I have provided figures ofdrawings in which:

FIG. 1 represents a plan view of the mechanical part of an embodiment ofmy invention;

FIG. 2 represents in elevation the embodiment represented in plan inFIG. 1, and schematically the electrical portion of such an embodiment;

FIG. 3 represents a convenient mode of application of an embodiment suchas that represented by FIGS. 1 and 2;

FIG. 4 represents an alternate mode of applicationof such an embodiment;and

FIG. 5 represents an embodiment of a simplified version of my invention,more restricted in its application than that represented by FIGS. 1 and2, but also less expensive.

FIG. 1 represents the plan view of an embodiment of my invention whichis shown partly in elevation and partly schematically in FIG. 2. A tankor bath 10 similar to that in the conventional Hull cell and hererepresented as being of transparent plastic material contains acounter-electrode 12 suitable for the deposition of a material whosedeposition is to be controlled. It is connected through rheostat 14 to acurrent source 16, here represented as a battery, which is connected byway of amrneter 18 to the test electrode. The test electrode (or, moresimply, electrode) is not a single piece of metal but consists of aplurality of strips 20, 22, 24, 26 and 28 which are carried upon asupply reel 30 which is supported by a shaft 32 which rotates inbearings 34 and 36. Counterelectrode 12 is is oriented at an angle tothe faces of electrode strips 20 through 28, inclusive, in order thatthe path lengths from each such strip to counterelectrode 12 may bedifferent, producing different current densities at each such strip whenthe potential difference between each such strip and thecounterelectrode 12 is the same. Ammeter 18 is represented as connectedto bearing 36. What is actually required in this instance is that theelectrical connection be made to all of the strips 20 through 28inclusive, and this is conveniently symbolized by connection to bearing36, it being assumed that shaft 32 and drum 30 are of metal and are inelectrical connection through bearing 36 with arnmeter 18. In practice amore formal slip ring and brush arrangement would probably be desirable,but since such a device is completely conventional, it is notrepresented here, purely to avoid complicating the drawing unprofitably.The various electrode strips pass from drum 3%) over sheave or roller 38into bath 39, pass around rollers 40 and 42 upward out of the bath overrollers 44 and 46, and are taken up by takeup roller 48, which iscarried by shaft 50 which rests in bearings or pillow blocks 52 and 54.No framework or mechanical structure is shown to support the variousbearings or, in the case of the rollers or sheaves, even supportingshafting. Such structure has been completely conventional for a numberof centuries and to represent it would complicate the drawing and rendermore difficult the explanation of the operation of my invention.

It is evident that in their passage through the bath 39, the variouscathode strips 20 through 28 will be plated by passage of current fromanode 12 through bath 39 to the electrode strips. It is also evidentthat while arnmeter 18 will give a correct reading of the total currentflowing through anode 12, the current density to the various cathodestrips will vary, strip 20 being plated at the high est current densityand strip 28 at the lowest. This effect is quite similar to the effectobserved in the Hull cell which,.however, conventionally employs a fixedsingle cathode and is used to determine only the appearance produced byelectrodeposition at various current densities.

In general, the stresses in electrodeposits are a function of thecurrent density. This function is, in general, not monotonic. However,for convenience in representing a variety of possible effects, I havechosen to represent the situation which would occur if the linear stressin the electrodeposit were in fact a monotone function of currentdensity, with high current density producing a deposit which is intension and low current density producing a deposit which is incompression. Thus, in FIG. 2, electrode strip 28 is represented as bowedout to the left in the direction of anode 12 by the compressive stressin a coating upon it. Electrode strip 24 is represented as being perfectly straight, an indication of the fact that the particular currentdensity at which it is plated produces a substantially stress-freedeposit. Electrode strip 20 is represented as bowed inward, or to theright, away from counterelectrode 12. Such bowing would be indicative ofa deposit in tensile stress. It is a purpose of my invention todetermine which of the electrode strips is operating, or being depositedat a current density which will produce a minimum of stress. For thispurpose, I have provided a plurality of position sensors, represented byrectangles 56 and supported by a bafiie 57. This insulating baffle 57serves an additional convenient purpose in that it will tend to minimizeany diversion of plating current to the portions of the electrode strips20 through 28 which have moved beyond roller 40 and are moving up fromroller 42 to roller 44. While a number of devices for measuring thedisplacement of a metal strip, such as one of the elec trode strips, areknown, a particularly convenient form for my purpose is an inductorwhich is located and so oriented that a displacement of the cathodestrip adjacent to it will cause its inductance and thus its reactance tovary. By including such an inductance as one of thefrequency-determining elements in a conventional oscillator circuit, theoutput frequencyof such an oscillator may be made to depend upon thedisplacement of the metal strip from the inductor. In FIG. 2, I haverepresented a plurality of such oscillators 58 which are connected by amulticonductor cable 59 individually to the appropriate sensors orinductors 56. Each such oscillator 58 is represented as connected to atuned detector circuit 69. This detector circuit may be a resonantcircuit which is tuned to give peak response at the oscillator frequencywhich is produced when the corresponding electrode strip is at its desired displacement from the corresponding inductor 56. In the presentcase, this desired displacement would correspond to the displacement ofstrip 24. Since displacement on either side of the desired frequencywill cause the output of a tuned circuit to decrease, it is evident thatthe oscillator 58.3 associated with strip 24 will be the strip uniquelytuned to match the tuning of its associated detector circuit 60.3 andtherefore that one tuned detector circuit will produce a higher outputvoltage than any of the other circuits. The outputs of the various tuneddetector circuits will feed through their buffer diodes 62 individuallyto the windings of relays 64- whose o posite winding terminals arebrought together and connected through a resistor 66 to ground, which isalso the common point for the various tuned detector circuits 60. Thefunctioning of this circuitry is as follows: Assuming that the tuneddetector 60.3, Which is associated with electrode strip 24, is producingan output voltage higher than that of any of the other tuned detectorcircuits, current will flow from it through its associated butter diode62.3 and the winding of its associated relay 64.3 through resistor 66 toground. If the resistance of resistor 66 is large compared with theresistance of the winding of a relay 64, the potential across resistor66 will be very nearly equal to the peak output potential of the tuneddetector circuit which is feeding it. Assuming an appreciable difierencebetween the output of the tuned detector circuit 60.3 associated withelectrode strip 24 and those of the other, somewhat detuned, detectorcircuits, the potential produced across resistor 66 will be greater thanthe output potentials of the other detector circuits, and the diodes 62associated with them will therefore be biased off. In other words, theonly tuned detector circuit which will actually be feeding current toresistor 66 and therefore the only tuned detector circuit which willactually be feeding current through a relay coil of relays 64 will bethe tuned detector circuit 60.3 which has the highest output. Thus, onlyone of the relays 64 will actually be excited at any given time. It willbe observed that the contacts of the relays 64 are so arranged that, ifnone of the relays is excited, the connection to terminal 68 ofcontrolled power supply 70 is simply carried through the normally closedcontacts of the relays back to the uppermost of the array 72 ofterminals on the power supply. If one of the relays is excited, it willbreak the connection to the uppermost of the array of terminals 72 andtransfer the connection with common terminal 68 to an intermediate oneof the array of termi nals 72. Controlled power supply 70 is required tobe a source of deposition current, connected by conductors 74 to theactual tank plating circuit and capable of being adjusted by connectionof common terminals 68 to a specific one of terminals 72, to furnish aspecific current to the electrodeposition circuit. In other words,connection of terminals 68 to a selected one of terminals 72 determinesthe particular current which the power supply will provide. Thiscorresponds, of course, to setting a tap switch to a particular point;but the convenience of using relays in this particular embodiment of myinvention requires that terminals rather than switch points he providedfor connection to the relay contacts.

It is evident that the results provided by the embodi ment of myinvention which have been described thus far will not bear anyparticular relation to the desired stress-free product unless thesolution or bath 39 em ployed in container 10 is representative of thedeposition medium whose product is to be controlled, and the currentdensity provided by power supply 70 corresponds to the current densitywhich produces the desired test result in the plating of one of theelectrode strips 20 through 28. The current density is, of course, afunction of the total current provided through conductors 74 and the ofthe area of work pieces in the main deposition tank at one time. Theseare design and operating parameters which obviously must be arranged forin the design of power supply '78 and the relationship provided betweenthe selection of a terminal of array 72 and the corresponding currentprovided through conductors 74. The problem of making bath 39representative of the medium actually used in the plating bath to becontrolled may be solved in one of two simple ways. The container 16 maybe perforated so that it may be mounted to support the electrodes andother structure in the main plating tank. to be served, in which case,the bath 39 will, of course, be part of the main plating bath. Such useof a Hull cell is completely conventional, and may also be the mostconvenient way of applying my invention. However, it is quite common inplating baths to provide circulating means so that the bath may becirculated continuously. If some circulating means for the entire bathis conveniently available, it may be more convenient to set up theembodiment represented in FIG. 1 and part of FIG. 2 independently of themain bath itself and simply provide for a portion of the circulated mainbath to be fed to container 10 and allowed to overflow back into themain electrolyte circuit from some convenient drain point. What isessential is that the bath 39 in container 10 shall be an accuratesample of the deposition medium actually being employed in the mainplating tank. The adequacy of the sample will, of course, take accountnot only of chemical composition but also of temperature.

Thus far, nothing has been said about the means for driving the cathodestrips 20 through 28 and the speed with which they should be fed. Sincethe object of coating the strips is to achieve a simulation of thecoating of the work pieces in the main plating tanks, the passage of thestrip from its entry into bath 39 to its reaching roller 40 shouldrequire a length of time equal to the time that the actual work piecesare in the main plating tanks. Deposition at a given current density fora given length of time will produce a fixed thickness of deposit. Thismeans that the deflection of the cathode strips will occur over a lengthof strip extending between roller 38 and roller 40* and, in consequence,will extend over a strip initially unplated and finally completelyplated. Thus, the total deflection of the cathode strip under theoperating conditions of the present invention will not be identical withthat which would result if the same length of cathode strip wereuniformly plated; but the deflection actually observed can be relatedexactly with the plating stress. The factors discussed thus will inpractice not impair the utility of the device. A further advantage ofdepositing, upon the electrode strip, substantially the same thicknessof deposit that will be applied to work pieces, is that, after itswithdrawal from the bath 39 and passage over roller 44, the deposit maybe examined visually or its thickness may be checked automatically byvarious known methods or other characteristics may be measured. Meansfor measuring these parameters are not shown in detail because they arewell known in the art and the single convenient variable parameter,current, is used in the present invention to control the stress in thedeposited coating. If examination of the deposit upon the strip after ithas been withdrawn from the bath on its way to takeup reel 48, shouldindicate that something in the nature of the coating is unsatisfactory,appropriate changes in bath composition or speed of traverse of the workthrough the main plating techniques may be made by human intervention inaccordance with the known art. It is a possible alternative to providefor intermittent advance of the electrode strips by controlling theoperation of motors 76 and 78 through a conventional time or clockswitch which operates them for a period suflicient to provide acompletely fresh electrode surface exposed between rollers 38 and 40,then stops the motors for a length of time equal to the length of timethat the work pieces are submitted to treatment, and then repeats itscycle once more. When this alternative mode of operation is employed,the properly significant value of deflection of the strips such as 20,24, and 28 will exist only at the end of the period during which themotors 76 and 78 are stopped. It is thus necessary, in this alternativemode, that the sensing system comprising 56, 58, 60, 62, and 64 berendered inactive (conveniently, by additional contacts on the timeswitch) at all time except at the end of that stop period, when thesensing system is briefly activated. In order that the value of platingcurrent provided through conductors 74 by controlled power supply 70shall not be altered except during activation of the sensing system,relays 64 may be of the conventional latching type which employs twocoils, one to latch the relay into a first position, and a second coilto unlatch the relay and lock it into its second position. In such case,each such latching relay must be equipped with an additional pair ofcontacts so that, when it is excited through its buffer diode 62 andlatched into its first position, the closing of the additional pair ofcontacts will excite the second coil of all the other relays to unlatchthem. This method is conventional, and other similar conventionaldevices are known and may be used. It will be recognized that, in thisalternative method, the speed of motors 76 and 78 may be as high asconvenient so that the time required to advance a fresh sample ofelectrode strip into position will be minimized.

The rotation of reels 3i and 48 to advance the electrodes isaccomplished by advancing means represented as motors 76 and 78 whichare represented as belted to the respective shafts 32 and 50. Since itis desired that the strips 20 through 28 be reasonably free to deflectunder their internal stresses while in transit from roller 38 to roller40, the tension upon these strips should not be excessive. Therefore, itappears desirable that the motor 76 be used as the speed determiningmotor and that the motor 7 8 be used to apply only suflicient torque toinsure that the strips pass over the various rollers and are wound up ontakeup reel 48 at moderate but not excessive tension. Since the tensionin the strips will be of very slight effect in reducing smalldeflections, it results that the tension in the strip is not extremelycritical; but excessive tension would reduce the sensitivity of thedevice to the appearance of stresses in the deposited. coating. This,obviously, is to be avoided.

FIG. 3 represent the structure of FIGS. 1 and 2 installed in a mainplating tank 80. In this mode of installation the tank 10 is providedwith apertures 82 in its walls so that the electrolyte 84 in the maintank may circulate freely into and outof tank 10, rendering its contentrepresentative of the content of the main plating tank 80. Details ofthe auxiliary equipment, shown in FIGS. 1 and 2, are omitted herebecause the reduced scale would render them undecipherable.

FIG. 4 represents an alternative mode of application of the structure ofFIGS. 1 and 2 to a main electrodeposition tank 86, which is providedwith a pump to recirculate the electrolyte 92. The dischargedelectrolyte from pump 90 is passed through a T94, whose outlets arethrottled by valves 96 and 98. By partially closing valve 96, a pressuredrop is created which will cause some of the circulating medium 92 topass through valve 98 (which may be adjusted to control the flow moreexactly) and thence into tank 10. Tank 10 for the present mode ofinstallation is located at a higher level than the level of medium 92 inthe main tank 86, and is provided with a floor drain 100 whichdischarges the medium 92 back into the main tank 86. In operation, valve98 may be adjusted to maintain a continuous flow of medium 92 throughthe tank 10 to preserve a suitable level in tank 10. Thus there ismaintained a continuously representative sample in tank 10 of the medium92 which is being used in the main tank 86.

The work pieces and electrodes in the main tanks 80 and 86 have not beenrepresented, since they are part of well known art, and may have a widevariety of embodiments.

It is evident that the basic principles of my invention are subject tomodification in accordance with the known art to meet particularrequirements. For example, it is possible to insulate the various testelectrode strips from each other and provide separate electricalconnections to each such strip. Then current may be fed separately viaeach separate electrical connection from independently adjustableseparate sources. In such an embodiment the various current densities tothe various electrode strips may be adjusted at will, without thenecessity of orienting the different strips so they will lie atdifferent distances from the counterelectrode. Such a procedure isnecessarily more complicated than the embodiment I have represented, andI have therefore not represented it as preferred; but it is feasible,should particular circumstances render it desirable. Similarly, I haverepresented the electrode strips as descending vertically through theregion in which deposition upon them takes place; but it is obviousthat, if it were desired, they might be caused to move horizontallythrough the bath, or at an angle. This ordinarily would have noadvantages, but is perfectly feasible. The embodiment which I haverepresented is my preference over these alternatives because of itssimplicity, which not only achieves economy, in general, but renders itmore easily understood by potential users.

While the foregoing embodiments of my invention have the advantage thatthey are applicable in all electrodeposition baths in which depositstress is a function of current density, itis possible to employ a stillsimpler embodiment for those baths in which, over the usual workingrange, stress is a monotone (whether increasing or decreasing) functionof current density. Such an embodiment is represented in FIG. 5. Since anumber of the elements represented in FIG. 5 are similar to, but notnecessarily identical with, elements represented in FIG. 2,

such similar elements in FIG. 5 are given reference num-,

bers 100 higher than their cognates in FIG. 2. Thus electrode 112 is thecognate of electrode 12.

In this simplified embodiment a single strip 128 is employed for testpurposes. It is plated at the average current density being employed toplate the work; and, after a suflicient time has elapsed to produce adeposit thick enough to deflect the strip 128 in a direction and amountindicative of the magnitude and sign of the stress in the deposit, suchdeflection is sensed by position sensor 156,

and the current to the plating circuit is adjusted in the direction(increase or decrease) which will alter the stress toward the value ofstress desired (which may be zero). Clearly, it is necessary, to permitthis procedure, that a given value of observed stress shall indicateuniquely whether the plating current should be increased or decreased;that is, the known relation between stress and plating current densitymust be monotone increasing or decreasing, since this means that thesign of the derivative never changes.

Such a relation is reported, for example, for the following copperelectroforming bath:

Copper sulfate oZ./gal 32 Sulphuric acid oz./gal 810 Temperature degreesC 35 Current density amps. sq. ft -40 Stress range lbs./sq. in 500 to+450() Referring in detail to FIG. 5, a counterelectrode 112 is immersedin bath 139. A supply reel 130, running on bearings 136, carries a storeof a single electrode strip 128 which runs over rollers 138, 140, 142,144, and 146 back to a takeup reel 148. Motor 178 draws the strip 128when it is to be moved, and motor 176 maintains moderate tension on it.The deflection of strip 128 is sensed by sensor 156, which is single,and is connected by cable 159 to oscillator 158 in some fashion (manyare known) which causes the frequency of the output of oscillator 158 tobe altered by variations in the deflection of strip 128. Oscillator 158has its output connected to a discriminator 160. While thisdiscriminator is the cognate of the tuned detectors 60 of FIG. 2, itdiffers from them in that it produces a D-C output which varies not onlyin magnitude but also in sign with the magnitude and sign of thedeflection of strip 128 from sensor 156. Barrier 157 serves the samefunction as does barrier 57 of FIG. 2. It'may be observed that, for abath of good throwing power, it may be desirable to provide the back ofstrip 128 with an insulating coating of cheap lacquer or similarmaterial to insure that only one side of the strip is plated. This willinsure maximum sensitivity of the stress observation. This is, ofcourse, well within the scope of the known art.

The embodiment of FIG. 5 has been chosen to illustrate specifically themode of operation in which the strip 128 is moved rapidly into position,plated, and its deflection is then measured and the plating current isadjusted accordingly. For this purpose, motors 176 and 17 8" arerepresented with their ungrounded driving power terminals connected by acommon conductor 202 to a contact segment 204 of a rotating timingswitch 206. Timing switch 206 comprises a clock motor 208 which drives arotating contact 210 clockwise over a circular path which comprisesinsulating segments 212 and 214 and contact segments 204 and 216.Rotating contact 210 is connected to a power source, not shown. Theproportionin-g of the various segments (and the speed of rotation ofclock motor 208i are such that the period of traverse of rotatingcontact 210 over insulating segment 212 is the length of timedesired forthe plating operation on strip 128. The subsequent period of contact ofrotating contact 210 with contact segment 216 is sufficient foradjustment of the plating current from plating power supply 170, bymeans to be described hereinafter. Following its contact with contactsegment 216, the rotating contact 210 remains on insulating segment 214for a brief period, which is determined primarily by the necessityof-preventing simultaneous contact between contact segments 216 and 204through the finite breadth of rotating contact 210. Then rotatingcontact 210 comes in'contact with contact segment 204, exciting motors176 and 178 for a period long enough to advance strip 128 by asufficient amount to present a fresh, unplated portion for plating. Theduration of this period will be determined primarily by the speed ofmotors 176 and 178, which may be as rapid as mechanical considerationspermit. Then, when rotating contact 208 moves to insulating segment 212,the motors stop and the fresh portion of strip 128 is plated.

The events which occur when rotating contact 210 is in contact withcontact segment 216 will now be considered in detail. Oscillator 158operates at a frequency which, as has been previously mentioned, is afunction of the displacement of strip 128 from sensor 156, and itsoutput is fed to discriminator 160, whose output is a D-C voltage whichis a function, in magnitude and sign, of the frequency of oscillator 158and thus of the displacement of strip 128' from sensor 156. One outputterminal of discriminator 160 is grounded, and the other output terminalis connected to an input terminal of amplifier 218, whose other inputterminal is connected to the movable contact of potentiometer 220, whichis fed a D-C potential bipolar with respect to ground from D-C source222. Amplifier 218 is a bipolar D-C amplifier whose Ol1tput capabilitiesare suitable for driving the armature of motor 224, which is representedas having a permanent field supplied by magnet 226, in order that thedirection of rotation of motor 224 may be dependent upon the polarity ofthe potential applied to its armature. The shaft of motor 224 isconnected to drive directly the movable contact of variable-ratioautotransformer 228, and, through electromagnetic clutch 227, themovable contact of potentiometer 220. Amplifier 218 has the additionalcharacteristic that it produces an output only when it is furnishedpower from contact segment 214 through conductor 230, which also engagesclutch 227. The mode of operation of the assemblage comprising amplifier218, motor 224, potentiometer 220 and autotransforrner 228 is asfollows: when amplifier 213 receives power over conductor 230 itproduces an output whose polarity and magnitude depend upon thedifference between the output signal of discriminator 160, and thepotential of the movable contact of potentiometer 220. The output signalof discriminator 160 is a measure of the desired corrective displacementof the movable contact of autotransformer 228. Spring 229, during theperiod prior to engagement of clutch 227 by excitation of conductor 23%,sets potentiometer 220 so its output is at ground potential. When clutch227 is engaged, subsequent rotation of 220 produces an output voltageproportional to the displacement of the contact of autotransformer 228.The output of amplifier 218 will therefore be proportional, in magnitudeand sign, to the diiference between these two values. The polarity ofthe connection of the output of amplifier 218 to the armature of motoris such as to cause motor 224 to rotate in such a direction as to reducethis diiference to zero-in other words, to displace the movable contactof autotransformer 228 by the desired amount. When rotating contact 210leaves contact segment 216, clutch 227 disengages itself and spring 229will center potentiometer 222, amplifier 218 will remain quiescent, andthe position of the movable contact of autotransformer 228 will remainundisturbed until the amplifier 218 is again rendered operative whenrotating contact 210, having completed a revolution again reachescontact segment 216. Thus, each time the plating operation on a portionof strip 128 has been completed, autotransformer 228 will be reset, ifnecessary, to the proper position to cause it to apply to plating powersupply the proper input potential to correct for any undesireddeflection sensed by sensor 156.

The direction of the changes produced by the described operation will bedetermined by the polarity of connections among the various elements.The general requirement which must be fulfilled is that the platingcurrent be changed in that direction which, from the known relationbetween stress and plating current (which must be initially determinedfor the particular bath and plating conditions employed, in accordancewith the known art), will tend to correct a sensed undesired deflectionof strip 128. Preferably, the magnitude of the change produced shouldbesuch as to reduce the undesired deflection to zero. In practice, thisis unlikely to be achieved exactly, but (since the period of rotation ofclock motor 208 may be made small compared with the time required forthe occurrence of substantial changes in the composition or otherplating conditions) frequent corrections will permit the values ofplating current to oscillate with small amplitude around the idealvalue, and any resulting stresses in the deposit produced will besatisfactorily small.

It should be observed that the particular system comprising amplifier218, motor 224, potentiometer 220, and its D-C source 222 is simply aparticular embodiment of a servo system which may be replaced inpractice by any servo system which can adjust the setting ofautotransformer 22% in accordance with the output of discriminator 160.Also, the signals resulting from the instrumental deflection sensing maybe electrical phaseor pulse-position-rather than frequencyor (at theoutput of the discriminator) amplitude-modulated. The most valid generalstatement one can make is that the signals will be functions of themagnitude and sign of the deflections sensed. Similarly, the use ofautotransformer 228 is merely exemplary, and it is perfectly feasible toreplace it and plating power supply 170 by any other device which can becaused to adjust the plating current furnished through conductors 174 inaccordance with the intermittent application of output signals receivedfrom discriminator 160. It should, however, be noted that the output ofdiscriminator 160 is properly significant of the plating conditions, andthe required current change, only at the completion of the plating of asample of strip 128; provision must therefore always be made, as in thepresent embodiment, to insure that the output of discriminator 160 iseffective only at the proper time in the cycle of operation.

It is, of course, feasible to operate a single-strip device of thegeneral kind represented by FIG. in the manner described in detail forthe embodiment of FIG. 2, in which the strip 123 slowly movescontinuously, so that the deflection sensed by sensor 156 is an averageindication of the history of the plating operation for the time requiredfor unrolling a length of strip 128 over a distance equal to the Spacingbetween rollers 138 and 140, and the plating current is adjustedcontinuously (or continually) in accordance with the sensed deflections.This latter method may be somewhat simpler but also may be somewhat lesssensitive.

Alternatively, it would be possible to provide successively exposedportions of strip electrode 128 with different current densities fordeposition, instrumentally measure the deflection resulting from eachcurrent density, store this information, and, upon completing a cycle ofmeasurements, determine the value of current density which most nearlyproduces the desired deposit stress, and adjust the current used inplating the workpieces to that value (or even interpolate between thetwo adjacent sampling current densities which produce the two values ofdeposit stress nearest to that desired). Those skilled in the art willrecognize this as being primarily an application of modern informationprocessing and control tech niques to achieve, by a single electrodestrip along the general lines disclosed by FIG. 5, the results taught bya plurality of electrode strips according to the disclosure of FIGS. 1and 2. Except under most unusual circumstances, it would appear to havelittle to recommend it. The period for complete sampling over the entirecurrent range will necessarily be extended over that required with aplurality of strips; the data processing and control equipment requiredmay be expected, at the present time, to exceed in cost the cost ofproviding for a plurality of strips. Furthermore, while the edge effectwhich causes current to be somewhat concentrated at the edges of a stripelectrode may be rendered negligible by making a single strip electrodesomewhat wider than would be needed for a plurality of strips, themutual edge shielding 12 effect of the plurality of strips is a benefitat least worth considering.

The appended claims are in subparagraph form in accordance with arequest of the Commissioner of Patents, to facilitate reading. Thedivision into subparagraphs is not necessarily indicative of relativeimportance of the elements recited, nor of any necessary relation amongsuch elements.

What is claimed is:

1. Monitoring means for an electrodeposition system composing:

(a) a bath representative of the deposition medium of the system to bemonitored;

(b) a counterelectrode immersed in said bath;

(c) a plurality of electrode strips;

(d) storage means for storing said electrode strips;

(e) support means for supporting said electrode strips for advance fromsaid storage means through said bath;

(f) advancing means for advancing said electrode strips on said supportmeans;

(g) current means for driving current between the said counterelectrodeand the said electrode strips at different current densities ondifferent electrode strips to produce electrodeposits upon saidelectrode strips;

(h) instrumental deflection sensing means to sense the deflection ofeach said electrode strip with respect to the said support means.

2. A device claimed in claim 1 further comprising:

(i) selection means connected to said instrumental deflection sensingmeans to select the electrode strip whose deflection most nearlyapproximates a desired deflection, and to actuate control meansresponsively thereto;

(j) control means connected to said selection means and to controlledpower supply means to control the current supplied by said supply meansresponsively to actuation by the said selection means.

3. Monitoring means for an electrodeposition system comprising:

(a) a plurality of extensive electrodes;

(b) counterelectrode means;

(c) means for supporting and advancing said plurality of extensiveelectrodes through a bath representative of the deposition medium of thesystem to be monitored, a portion of each said electrode in the saidbath being supported only at the extremes of the said portion;

(d) current means for providing current flowbetween the said bath andthe said electrodes at diflierent current densities for differentelectrodes to produce an electrodeposit upon the said electrodes;

(e) instrumental deflection sensing means for sensing the deflection ofeach said portion from a straight line between the ends of the saidportion.

4. Monitoring means as claimed in claim 3 further comprising:

(f) means for controlling an electrodeposition current responsively tothe deflections sensed by the said instrumental sensing means.

5. Monitoring means as claimed in claim 4 in which the said means forcontrolling an electrodeposition current comprises:

(1) means for selecting the said strip whose deflection as sensed by thesaid instrumental deflection sensing means is most nearly zero;

(2) means for controlling the said electrodeposition current to beproportional to the current density existing at the said selected strip.

6. In combination:

(a) a bath of electrodeposition material;

(b) a counterelectrode immersed in said bath;

(c) a plurality of electrode strips;

((1) a supply reel for storing a supply of said plurality of electrodestrips;

(e) a plurality of support rollers for carrying said electrode stripsfrom said supply reel into and out of said bath;

(f) a takeup reel for taking up the said electrode strips after passageover the said support rollers;

(g) advance means to advance the said electrode strips from the saidsupply reel over the said support rollers to the said takeup reel;

(h) current supply means connected to provide current between the saidcounterelectrode and the said plurality of electrode strips atdiflFerent current densities to different electrode strips of the saidplurality, to cause electrodeposition upon said electrode strips;

(i) a plurality of instrumental deflection sensing devices, equal innumber to the number of said electrode strips, located in the said batheach adjacent to the portion of an electrode strip extending between twoof said support rollers, to sense the deflection of the said portion ofa said electrode strip from a straight line tangent to the said twosupport rollers.

7. The combination claimed in claim 6, further combined with:

(j) selecting means for selecting from the said plurality of deflectionsensing devices the deflection sensing device which is sensing asubstantially zero deflection, and

(k) means for adjusting the current output of a con trolled power supplyto a value proportional to the current density provided to the saidelectrode strip selected by the said selecting means.

8. The combination claimed in claim 6, in which a said instrumentaldeflection sensing device comprises:

(1) an electrical reactance element whose reactance is a function of thedeflection to be sensed;

(2) an oscillator whose frequency is determined in part by the reactanceof the said electrical reactance element;

(3) a frequency-selective circuit connected to receive the outputfrequency of the said oscillator.

9. The combination claimed in claim 7, in which the said selecting meanscomprises:

(1) a plurality of diodes connected to the outputs of the saidinstrumental deflection sensing means, and

(2) a common resistance connected to receive the output of any of thesaid diodes of the said plurality.

10. The combination claimed in claim 9, in which each diode of thetherein said plurality is connected to the therein said commonresistance through the winding of a relay whose contacts are connectedto control the current output of a controlled power supply.

11. Monitoring means for an electrodeposition system comprising:

(a) a bath representative of the deposition medium of the system to bemonitored;

(b) a counterelectrode immersed in said bath;

(c) an electrode strip;

(d) storage means for storing said electrode strip;

(e) support means for supporting said electrode strip for advance fromsaid storage means through said bath;

(f) advancing means for advancing said electrode strip on said supportmeans;

(g) current means for driving current between the said counterelectrodeand the said electrode strip to produce an electrodeposit upon saidelectrode strip;

(h) instrumental deflection sensing means to sense the deflection ofsaid electrode strip with respect to the said support means.

12. A device claimed in claim 11 further comprising:

(i) control means connected to said instrumental deflection sensingmeans and to power supply means to control the current supplied by saidsupply means responsively to signals which are functions of themagnitude and sign of the deflection sensed by said instrumentaldeflection sensing means.

13. A device claimed in claim 12 further comprising:

(j) timing means connected to cause said advancing means to operateintermittently during first periods of time, and to cause said controlmeans to respond to said signals intermittently during second periods oftime.

References Cited UNITED STATES PATENTS 2,762,763 9/1956 Kenmore et al204206 2,868,702 1/1959 Brennan 204206 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,356,605 December 5, 1967 Francis J. Schmidt It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the said Letters Patent should read as corrected below.

Column 12, line 11, for "composing" read comprising Signed and sealedthis 7th day of January 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissionerof Patents

11. MONITORING MEANS FOR AN ELECTRODEPOSITION SYSTEM COMPRISING: (A) ABATH REPRESENTATIVE OF THE DEPOSITION MEDIUM OF THE SYSTEM TO BEMONITORED; (B) A COUNTERELECTRODE IMMERSED IN SAID BATH; (C) ANELECTRODE STRIP; (D) STORAGE MEANS FOR STORING SAID ELECTRODE STIP; (E)SUPPORT MEANS FOR SUPPORTING SAID ELECTRODE STRIP FOR ADVANCE FROM SAIDSTORAGE MEANS THROUGH SAID BATH; (F) ADVANCING MEANS FOR ADVANCING SAIDELECTRODE STRIP ON SAID SUPPORT MEANS; (G) CURRENT MEANS FOR DRIVINGCURRENT BETWEEN THE SAID COUNTERELECTRODE AND THE SAID ELECTRODE STRIPTO PRODUCE AN ELECTRODEPOSIT UPON SAID ELECTRODE STRIP;